<!DOCTYPE html>
<html lang="en" style="font-size: 12px;" >
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 200px;
            height: 400px;
            font-size: 16px;
            background-color: #ccc;
        }
        /* r root */
        .child{
            width: 100px;
            height: 100px;
            margin-top: 10%;
            padding-top: 10px;
            /* background-color: #eee;
            border: 1rem solid red; */
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="child"></div>
    </div>
</body>
</html>